This invention relates to a thermistor with resistance having a positive temperature coefficient, or a so-called PTC thermistor. In particular, this invention relates to a PTC thermistor with improved resistance against flash pressure.
PTC thermistors are required to have a large resistance against flash pressure when used for protection against an over-current, for demagnetization or in a motor starter. FIGS. 3A and 3B show a typical prior art PTC thermistor 1 having electrodes 6 and 7 individually formed on the mutually oppositely facing main surfaces 3 and 4 of a circular disk-shaped main body 2. Numeral 5 indicates the side surface of this disk-shaped main body 2. FIGS. 4A and 4B show another prior art PTC thermistor 11 which also has electrodes 16 and 17 individually formed on the mutually oppositely facing main surfaces 13 and 14 of a circular disk-shaped main body 12 but is different from the example shown in FIGS. 3A and 3B in that the main body 12 is divided into three regions in the direction of its thickness, that is, a center region 18 and two outer regions 19 and 20 which sandwich it in between, the outer regions 19 and 20 having a higher specific resistance than the inner region 18. Such an prior art PTC thermistor has been disclosed in Japanese Patent Publication Tokkai 9-17606. In FIGS. 4A and 4B, numeral 15 indicates a side surface of the main body 2, extending in the direction of the thickness and connecting the circular peripheries of the two main surfaces 13 and 14.
When a potential difference is applied between the electrodes 6 and 7 of the PTC thermistor 1 shown in FIGS. 3A and 3B, its main body 2 begins to generate heat. During the initial stage of its heat emission, the region of peak heat emission is at the center of the main body 2 in the direction of its thickness. As a result, the temperature distribution inside the main body 2 in the direction of its thickness becomes as shown in FIG. 3C. Thus, a relatively large tensile force is generated and the main body 2 is likely to be damaged if its resistance against flash pressure is not sufficiently strong.
When a potential difference is applied between the electrodes 16 and 17 of the PTC thermistor 11 shown in FIGS. 4A and 4B, on the other hand, two peak heat emission regions appear inside its main body 12 during its initial stage of heat emission. As a result, the temperature distribution inside the main body 12 in the direction of its thickness becomes as shown in FIG. 4C. In other words, the two temperature peaks are reasonably well separated and the overall temperature distribution is better balanced.
In spite of the advantage described above, the PTC thermistor 11 shown in FIGS. 4A and 4B are more troublesome and more costly to manufacture because two different materials must be used to manufacture its main body 12 and an extra step is involved for forming a layered structure.